Limit-level sensor and method of determining the undershooting or overshooting of an intended level of liquid in a container

ABSTRACT

A limit-level sensor ( 3 ) has a resistance element ( 4 ) which has an electric resistance which suddenly varies a transition temperature which lies above the maximum liquid temperature. The resistance element ( 4 ) is first of all heated by electric current. Thereupon, the electric resistance of the resistance element ( 4 ) is measured. When the resistance element ( 4 ) is covered with liquid ( 2 ), the heat generated by the electric current is led away so that the temperature of the resistance element ( 4 ) is less than its transition temperature. If the resistance element ( 4 ) is not covered by liquid ( 2 ), this heat is scarcely led away, so that the temperature of the resistance element ( 4 ) remains above the transition temperature.

[0001] The present invention relates to a level sensor which isdeveloped so as to determine the undershooting or overshooting of apredetermined level of liquid in a container, the sensor having aresistance element which is traversed by electric current and changesits electrical resistance upon an increase in the temperature, andhaving a measuring unit for measuring the electric resistance of theelectrical conductor. The invention also relates to a method ofdetermining the undershooting or overshooting of an intended level ofliquid in a container, in which method a resistance element which isarranged at the intended level of filling and changes its electricalresistance with an increase in temperature is heated by an electriccurrent whereupon its electrical resistance is measured.

[0002] Such limit-level sensors are frequently used in motor vehicles,for instance in a transmission, an internal combustion engine or a fueltank and are known from actual practice. As resistance element, thelimit-level sensor has a resistance wire which extends down into theregion of the bottom of a container to be measured. For the measurement,the resistance wire is traversed by an electric current. In this way,the resistance wire is heated, thereby changing its electricalresistance. Since the heat produced thereby in the region of theresistance wire immersed in the liquid is substantially more stronglyled away from the liquid than the region above the liquid, the level ofthe liquid can be calculated from the resistance ascertained andcompared with a minimum level.

[0003] This method has the disadvantage that the resistance of theresistance wire which is determined is influenced by the temperature ofthe liquid. The determination of the level is thus relativelyinaccurate. Therefore, a measurement of the minimum level of motor oilin an internal combustion engine of a motor vehicle having this levelsensor is possible only at great expense.

[0004] For the dependable determination of the level of a liquid havinga particularly high temperature, floats provided with a magnet whichfloat on the liquid are known. When an intended level is undershot orovershot, the magnet switches a reed contact which is arranged at theheight corresponding to the level. Such a limit-level switch, however,is particularly expensive. Furthermore, this level switch has movableparts which can jam.

[0005] The object of the present invention is to develop a limit-levelsensor of the aforementioned type in such a manner that it has nomovable parts and that the overshooting or undershooting of a level of aliquid can be reliably determined with it even at high temperatures.Furthermore, a method is to be provided for determining theundershooting or overshooting of an intended height of filling of liquidin a container.

[0006] The first-mentioned problem is solved in accordance with theinvention in the manner that the resistance element is arrangedexclusively at the intended height of filling and has an electricalresistance which changes suddenly above a transition temperature whichlies above the maximum temperature of the liquid.

[0007] The resistance element is first of all heated electrically likethe resistance wire of the known level-limit sensor. If the heatproduced thereby is removed very rapidly, the resistance element issufficiently covered by liquid. Since air removes the heat very poorlyfrom the resistance element, the resistance element will heat up to atemperature above the transition temperature (for instance a PTCelement) in case of too low a level of filling and initially retain thistemperature. In this way, a resistance value which has been determinedcan be ascribed unequivocally to a temperature range of the resistanceelement which lies above or below the transition temperature and anovershooting or undershooting of the level of the liquid can thus bedetermined. Since the temperature of the liquid lies below thetransition temperature of the resistance element, its influence on theresult of the measurement is negligible.

[0008] In accordance with an advantageous further development of theinvention, the limit-level sensor is suitable for use at particularlyhigh temperatures, if the resistance element is a PTC element. Such aPTC element has a particularly high transition temperature of about 250°C. Up to the transition temperature, the electrical resistance of thePTC element decreases slightly with an increase in temperature. Abovethe transition temperature, the electrical resistance increasesabruptly.

[0009] The electrical resistance of the resistance element can bedetermined in a particularly simple manner, in accordance with anadvantageous further development of the invention, if the resistanceelement is connected in series with a series resistor. In this way, thevoltage is measured on the resistance element. With the total voltage onthe series resistor and the resistance element known, the voltagemeasured is dependent directly on the electrical resistance of theresistor element.

[0010] The mounting of the resistance element is particularly simple inaccordance with another advantageous development of the invention if thelimit-level sensor has a housing which can be assembled from two housingparts and if connecting wires leading to the resistance element areclamped between the two housing parts.

[0011] A discharge of the heat produced by the heating of the resistanceelement is reliably prevented, in accordance with another advantageousdevelopment of the invention, if the resistance element is soldered orwelded to the connecting wires, and if the housing has exclusivelypoints of contact with the connecting wires. Furthermore, thisdevelopment has the advantage that liquid or can flow freely around theresistance element.

[0012] In accordance with another advantageous feature of the invention,the housing can be produced particularly inexpensively if one of thehousing parts has a plurality of thin-wall transverse ribs which havegrooves to receive the connecting wires and if the second housing partshas projections which lie on the connecting wires. Furthermore, by thisdevelopment, the removal of heat via the connecting wires and thehousing is kept particularly slight.

[0013] Particularly at low temperatures and with too low a level, a dropof liquid adhering between the resistance element and the housing canlead the heat away from the resistance element and result in anerroneous measurement. Such an erroneous measurement can be dependablyavoided in accordance with another advantageous feature of the inventionif there is a distance between the resistance element and the regions ofthe housing which are adjacent it.

[0014] The resistance element which is held exclusively by theconnecting wires requires particularly strong connecting wires for anoscillation-resistant attachment. A discharge of the heat of theresistance element over the connecting wires is kept particularly slightin accordance with another advantageous development of the invention ifthe connecting wires are made, for instance, of constantan.

[0015] A minimum level and a maximum level can be easily determined byanother advantageous development of the invention if the limit-levelsensor has two resistance elements arranged at different heights.

[0016] The second problem, namely the creation of a method for thedetermining an undershooting or overshooting of an intended level ofliquid in a container is solved, in accordance with the invention, inthe manner that upon the heating, the resistance element is fed as muchelectrical energy as is necessary in order to exceed a transitiontemperature of the resistance element present in air which abruptlychanges the electrical resistance.

[0017] By this method, a value which can be unambiguously associatedwith the region above or below the transition temperature, is obtainedupon the measurement of the electrical resistance of the resistanceelement. If the resistance element extends into the liquid, then theheat produced by the electric current is led away by the liquid and theresistance element has a temperature, and thus a corresponding value ofresistance, below the transition temperature. In the event that theresistance element does not extend into the liquid, the heat is not ledaway, so that the temperature of the resistance element remains abovethe transition temperature. A further advantage of this method is thatliquid which adheres to the resistance element is first of all alsoheated. This leads, for instance in the event of cold oil, to areduction in its viscosity and thus to a dripping of oil from theresistance element. In this way, erroneous measurements are reliablyprevented.

[0018] The method of measurement requires particularly little time inaccordance with an advantageous further development of the invention ifthe resistance element is heated with a electric current via a seriesresistance and the voltage then measured on the resistance element. Thevoltage measured on the resistance element is dependent on theelectrical resistance of the resistance element.

[0019] A limit-level sensor used for the determination of a minimumlevel of motor oil in an internal combustion engine of a motor vehicleis particularly reliable if the heating of the resistance element iscommenced about five to ten minutes after the disconnecting of theinternal combustion engine.

[0020] The invention permits of numerous embodiments. For furtherexplanation of its basic principle, one of these embodiments is shown inthe drawing and will be described below. In the drawing

[0021]FIG. 1 shows a limit-level sensor in accordance with theinvention, with a diagrammatic showing of an electric circuit;

[0022]FIG. 2 shows the limit-level sensor of FIG. 1;

[0023]FIG. 3 is a sectional view through the limit-level sensor of FIG.2 along the line III-III; and

[0024]FIG. 4 is a cross-sectional showing through the limit-level sensorof FIG. 2, along the line IV-IV.

[0025]FIG. 1 shows a limit-level sensor 3 in a container 1 containingliquid 2. The limit-level sensor 3 is provided on its lower end with aresistance element 4 which dips into the liquid 2 and is fed via aseries resistor 6 with electric current from a source of voltage 5arranged outside the container 1. The resistance element 4 can, forinstance, be a PTC element, in which the electric resistance decreasesslightly with an increase in the temperature until reaching a transitiontemperature which is a function of the material and increases suddenlyabove the transition temperature. The electric resistance of theresistance element 4 is determined by a voltmeter 7 which measures avoltage present on a resistance element 4. When the transitiontemperature of the resistance element 4 is exceeded, the voltagemeasured by the voltmeter 7 increases suddenly. Since the liquid 2 coolsthe resistance element 4 substantially more than air which is presentover the liquid 2, it can be determined in this way whether theresistance element 4 is covered with liquid 2.

[0026]FIG. 2 shows the limit-level sensor 3 of FIG. 1 with theresistance element 4 arranged in the lower region of a housing 8. Fromthe resistance element 4, connecting wires 9 lead to connecting contacts10 protruding from the top of the housing 8. The housing 8 has a thread11 and a hexagon 12, arranged above the tread 11 for attaching a wrench,not shown in the drawing. Furthermore, the housing 8 is provided on thebottom of the hexagon 12 with a packing ring 13 which is provided inorder to seal-off the limit-level sensor 3 which is mounted in thecontainer 1. In its lower region, the housing 8 has longitudinal bars 14and transverse bars 15 which are at a distance from the resistanceelement 4. The resistance element 4 is thus held in a cage which istraversed by the liquid 2 shown in FIG. 1.

[0027] The construction of the housing 8 and the attachment of theconnecting wires 9 of the limit-level sensor 3 is shown in FIG. 3. Thehousing 8 has two housing parts 16, 17 which are connected to each otherby detents 18. Transverse bars 15, 15 a are arranged on the housingparts 16, 17 respectively. The transverse bar 15 of the one housing part16 has grooves 19 to receive the connecting wires 9, while thetransverse bar 15 a of the other housing part 17 has projections 20which press the connecting wires 9 into the grooves 19. In this way, theconnecting wires 9 of the resistance element 4 are clamped between thetransverse bars 15, 15 a arranged on the two housing parts 16, 17 of thehousing 8.

[0028]FIG. 4 shows, in a sectional view through the housing 8, that theresistance element 4 is of disk shape and is fastened exclusively to theconnecting wires 9. In this way, the resistance element 4 is held fixedagainst oscillation and the liquid 2 flows around all of it.

1. A limit-level sensor which, for determining whether an intended levelof liquid in a container is not reached or is exceeded, has a resistanceelement which is traversed by electric current and changes itsresistance upon an increase in the temperature and has a measurementunit for measuring the electric resistance of the electric conductor,characterized by the fact that the resistance element (4) is arrangedentirely within the intended level and has an electric resistance whichvaries abruptly above a transition temperature which lies above themaximum temperature of the liquid.
 2. A limit-level sensor according toclaim 1 , characterized by the fact that the resistance element (4) is aPTC element.
 3. A limit-level sensor according to claim 1 ,characterized by the fact that the resistance element (4) is connectedin series with a series resistor (6).
 4. A limit-level sensor accordingto claim 1 , characterized by the fact that it has a housing (8) whichcan be assembled from two housing parts (16, 17), and that connectingwires (9) leading to the resistance element (4) are clamped between thetwo housing parts (16, 17).
 5. A limit-level sensor according to claim 4, characterized by the fact that the resistance element (4) is solderedor welded to the connecting wires (9), and that the housing (8) has onlypoints of contact with the connecting wires (9).
 6. A limit-level sensoraccording to claim 4 , characterized by the fact that one of the housingparts (16) has several thin-wall transverse bars (15) which have grooves(19) to receive the connecting wires (9), and that the second housingpart (17) has projections (20) which rest against the connecting wires(9).
 7. A limit-level sensor according to claim 4 , characterized by thefact that a distance is present between the resistance element (4) andthe regions of the housing (8) which are adjacent to it.
 8. Alimit-level sensor according to claim 4 , characterized by the fact thatthe connecting wires (9) are made of constantan.
 9. A limit-level sensoraccording to claim 1 , characterized by the fact that it has tworesistance elements (4) at different levels.
 10. A method fordetermining whether an intended level of liquid in a container isexceeded or not reached, a resistance element which is arranged withinthe intended level and changes its electrical resistance with increasein the temperature being heated by an electric current whereupon itselectric resistance is measured, characterized by the fact that as muchelectrical energy is fed to the resistance element upon the heating asis necessary in order to exceed a transition temperature of a resistanceelement present in the air which abruptly changes its electricalresistance.
 11. A method according to claim 10 , characterized by thefact that the resistance element is heated with electric current via aseries resistance and thereupon the voltage on the resistance element ismeasured.
 12. A method according to claim 10 for determining a minimumlevel of motor oil in an internal combustion engine of a motor vehicle,characterized by the fact that the heating of the resistance element iscommenced after the connecting of the ignition and a few seconds beforethe starting of the internal combustion engine or about five to tenminutes after a disconnecting of the internal combustion engine.